Michel Vivaudou

The molecular basis of the specificity of action of KATP channel openers

KATP channels incorporate a regulatory subunit of the ATP‐binding cassette (ABC) transporter family, the sulfonylurea receptor (SUR), which defines their pharmacology. The therapeutically important K+ channel openers (e.g. pinacidil, cromakalim, nicorandil) act specifically on the SUR2 muscle isoforms but, except for diazoxide, remain ineffective on the SUR1 neuronal/pancreatic isoform. This SUR1/2 dichotomy underpinned a chimeric strategy designed to identify the structural determinants of opener action, which led to a minimal set of two residues within the last transmembrane helix of SUR. Transfer of either residue from SUR2A to SUR1 conferred opener sensitivity to SUR1, while the reverse operation abolished SUR2A sensitivity. It is therefore likely that these residues form part of the site of interaction of openers with the channel. Thus, openers would target a region that, in other ABC transporters, is known to be tightly involved with the binding of substrates and other ligands. This first glimpse of the site of action of pharmacological openers should permit rapid progress towards understanding the structural determinants of their affinity and specificity.

Skeletal muscle ATP-sensitive K+ channels recorded from sarcolemmal blebs of split fibers: ATP inhibition is reduced by magnesium and ADP.

SummaryA new, nonenzymatically treated preparation of amphibian sarcolemmal blebs has been used to study the regulation of skeletal muscle ATP-sensitive K+ [K(ATP)] channels.When a frog skeletal muscle fiber is split in half in a Ca2+-free relaxing solution, large hemispherical membrane blebs appear spontaneously within minutes without need for Ca2+-induced contraction or enzymatic treatment. These blebs readily formed gigaseals with patch pipettes, and excised inside-out patches were found to contain a variety of K+ channels. Most prominent were K(ATP) channels similar to those found in the surface membrane of other muscle and nonmuscle cells. These channels were highly selective for K+, had a conductance of ≈ 53 pS in 140mmK+, and were blocked by internal ATP. The presence of these channels in most patches implies that split-fiber blebs are made up, at least in large part, of sarcolemmal membrane.In this preparation, K(ATP) channels could be rapidly and reversibly blocked by glibenclamide (0.1–10 μm) in a dose-dependent manner. These channels were sensitive to ATP in the micromolar range in the absence of Mg. This sensitivity was noticeably reduced in the presence of millimolar Mg, most likely because of the ability of Mg2+ ions to bind ATP. Our data therefore suggest that free ATP is a much more potent inhibitor of these channels than MgATP. Channel sensitivity to ATP was significantly reduced by ADP in a manner consistent with a competition between ADP, a weak inhibitor, and ATP, a strong inhibitor, for the same inhibitory binding sites.These observations suggest that the mechanisms of nucleotide regulation of skeletal muscle and pancreatic K(ATP) channels are more analogous than previously thought.

Zinc is both an intracellular and extracellular regulator of KATP channel function

Extracellular Zn2+ has been identified as an activator of pancreatic KATP channels. We further examined the action of Zn2+ on recombinant KATP channels formed with the inward rectifier K+ channel subunit Kir6.2 associated with either the pancreatic/neuronal sulphonylurea receptor 1 (SUR1) subunit or the cardiac SUR2A subunit. Zn2+, applied at either the extracellular or intracellular side of the membrane appeared as a potent, reversible activator of KATP channels. External Zn2+, at micromolar concentrations, activated SUR1/Kir6.2 but induced a small inhibition of SUR2A/Kir6.2 channels. Cytosolic Zn2+ dose‐dependently stimulated both SUR1/Kir6.2 and SUR2A/Kir6.2 channels, with half‐maximal effects at 1.8 and 60 μm, respectively, but it did not affect the Kir6.2 subunit expressed alone. These observations point to an action of both external and internal Zn2+ on the SUR subunit. Effects of internal Zn2+ were not due to Zn2+ leaking out, since they were unaffected by the presence of a Zn2+ chelator on the external side. Similarly, internal chelators did not affect activation by external Zn2+. Therefore, Zn2+ is an endogenous KATP channel opener being active on both sides of the membrane, with potentially distinct sites of action located on the SUR subunit. These findings uncover a novel regulatory pathway targeting KATP channels, and suggest a new role for Zn2+ as an intracellular signalling molecule.

Coupling ion channels to receptors for biomolecule sensing.

Nanoscale electrical biosensors are promising tools for diagnostics and high-throughput screening systems. The electrical signal allows label-free assays with a high signal-to-noise ratio and fast real-time measurements. The challenge in developing such biosensors lies in functionally connecting a molecule detector to an electrical switch. Advances in this field have relied on synthetic ion-conducting pores and modified ion channels that are not yet suitable for biomolecule screening. Here we report the design and characterization of a novel bioelectric-sensing platform engineered by coupling an ion channel, which serves as the electrical probe, to G-protein-coupled receptors (GPCRs), a family of receptors that detect molecules outside the cell. These ion-channel-coupled receptors may potentially detect a wide range of ligands recognized by natural or altered GPCRs, which are known to be major pharmaceutical targets. This could form a unique platform for label-free drug screening.

Modification by protons of frog skeletal muscle KATP channels: effects on ion conduction and nucleotide inhibition.

1. The molecular mechanisms underlying pH regulation of skeletal muscle ATP‐sensitive K+ (KATP) channels were studied using the patch clamp technique in the inside‐out configuration. Two effects of intracellular protons were studied in detail: the decrease in magnitude of single‐channel currents and the increase in open probability (Po) of nucleotide‐inhibited channels. 2. The pH dependence of inward unit currents under different ionic conditions was in poor agreement with either a direct block of the pore by protons or an indirect proton‐induced conformational change, but was compatible with the protonation of surface charges located near the cytoplasmic entrance of the pore. This latter electrostatic mechanism was modelled using Gouy‐Chapman‐Stern theory, which predicted the data accurately with a surface charge density of about 0.1 negative elementary charges per square nanometre and a pK (pH value for 50% effect) value for protonation of these charges of 6.25. The same mechanism, i.e. neutralization of negative surface charges by cation binding, could also account for the previously reported reduction of inward unit currents by Mg2+. 3. Intracellular alkalization did not affect Po of the KATP channels. Acidification increased Po. In the presence of 0.1 mM ATP (no Mg2+), the channel activation vs. pH relationship could be fitted with a sigmoid curve with a Hill coefficient slightly above 2 and a pK value of 6. This latter value was dependent on the ATP concentration, decreasing from 6.3 in 30 microM ATP to 5.3 in 1 microM ATP. 4. Conversely, the channel inhibition vs. ATP concentration curve was shifted to the right when the pH was lowered. At pH 7.1, the ATP concentration causing half‐maximal inhibition was about 10 microM. At pH 5.4, it was about 400 microM. The Hill coefficient values remained slightly below 2. Similar effects were observed when ADP was used as the inhibitory nucleotide. 5. These results confirm that a reciprocal competitive link exists between proton and nucleotide binding sites. Quantitatively, they are in full agreement with a steady‐state model of a KATP channel possessing four identical protonation sites (microscopic pK, 6) allosterically connected to the channel open state and two identical nucleotide sites (microscopic ATP dissociation constant, approximately 30 microM) connected to the closed state.

Adaptive torsion-angle quasi-statics

MOTIVATION The cost of molecular quasi-statics or dynamics simulations increases with the size of the simulated systems, which is a problem when studying biological phenomena that involve large molecules over long time scales. To address this problem, one has often to either increase the processing power (which might be expensive), or make arbitrary simplifications to the system (which might bias the study). RESULTS We introduce adaptive torsion-angle quasi-statics, a general simulation method able to rigorously and automatically predict the most mobile regions in a simulated system, under user-defined precision or time constraints. By predicting and simulating only these most important regions, the adaptive method provides the user with complete control on the balance between precision and computational cost, without requiring him or her to perform a priori, arbitrary simplifications. We build on our previous research on adaptive articulated-body simulation and show how, by taking advantage of the partial rigidification of a molecule, we are able to propose novel data structures and algorithms for adaptive update of molecular forces and energies. This results in a globally adaptive molecular quasi-statics simulation method. We demonstrate our approach on several examples and show how adaptive quasi-statics allows a user to interactively design, modify and study potentially complex protein structures.

Three C-terminal residues from the sulphonylurea receptor contribute to the functional coupling between the KATP channel subunits SUR2A and Kir6.2

Cardiac ATP‐sensitive potassium (KATP) channels are metabolic sensors formed by the association of the inward rectifier potassium channel Kir6.2 and the sulphonylurea receptor SUR2A. SUR2A adjusts channel gating as a function of intracellular ATP and ADP and is the target of pharmaceutical openers and blockers which, respectively, up‐ and down‐regulate Kir6.2. In an effort to understand how effector binding to SUR2A translates into Kir6.2 gating modulation, we examined the role of a 65‐residue SUR2A fragment linking transmembrane domain TMD2 and nucleotide‐binding domain NBD2 that has been shown to interact with Kir6.2. This fragment of SUR2A was replaced by the equivalent residues of its close homologue, the multidrug resistance protein MRP1. The chimeric construct was expressed in Xenopus oocytes and characterized using the patch‐clamp technique. We found that activation by MgADP and synthetic openers was greatly attenuated although apparent affinities were unchanged. Further chimeragenetic and mutagenetic studies showed that mutation of three residues, E1305, I1310 and L1313 (rat numbering), was sufficient to confer this defective phenotype. The same mutations had no effects on channel block by the sulphonylurea glibenclamide or by ATP, suggesting a role for these residues in activatory – but not inhibitory – transduction processes. These results indicate that, within the KATP channel complex, the proximal C‐terminal of SUR2A is a critical link between ligand binding to SUR2A and Kir6.2 up‐regulation.

Engineering of an artificial light-modulated potassium channel

Ion Channel-Coupled Receptors (ICCRs) are artificial receptor-channel fusion proteins designed to couple ligand binding to channel gating. We previously validated the ICCR concept with various G protein-coupled receptors (GPCRs) fused with the inward rectifying potassium channel Kir6.2. Here we characterize a novel ICCR, consisting of the light activated GPCR, opsin/rhodopsin, fused with Kir6.2. To validate our two-electrode voltage clamp (TEVC) assay for activation of the GPCR, we first co-expressed the apoprotein opsin and the G protein-activated potassium channel Kir3.1F137S (Kir3.1*) in Xenopus oocytes. Opsin can be converted to rhodopsin by incubation with 11-cis retinal and activated by light-induced retinal cis→trans isomerization. Alternatively opsin can be activated by incubation of oocytes with all-trans-retinal. We found that illumination of 11-cis-retinal-incubated oocytes co-expressing opsin and Kir3.1* caused an immediate and long-lasting channel opening. In the absence of 11-cis retinal, all-trans-retinal also opened the channel persistently, although with slower kinetics. We then used the oocyte/TEVC system to test fusion proteins between opsin/rhodopsin and Kir6.2. We demonstrate that a construct with a C-terminally truncated rhodopsin responds to light stimulus independent of G protein. By extending the concept of ICCRs to the light-activatable GPCR rhodopsin we broaden the potential applications of this set of tools.

Modulation by Mg2+ and ADP of ATP-sensitive potassium channels in frog skeletal muscle

SummaryThe patch-clamp technique was used to examine the action of intracellular magnesium ions and ADP in the absence of ATP on skeletal muscle ATP-sensitive potassium channels (K-ATP channels). Inside-out patches were excised from the membrane of sarcolemmal blebs which arise spontaneously without enzymatic treatment after a frog muscle fiber is split in half.In the absence of nucleotides, K-ATP channel open probability was not significantly affected by intracellular magnesium even at a concentration (20 mm) which fully blocks cardiac and pancreatic K-ATP channels. On the other hand, Mg2+ ions (10–20 mm) decreased both inward and outward unitary currents. The percent reduction in inward currents (about 8%) was independent of voltage while the reduction in outward currents was larger at higher voltages, suggesting that the former effect resulted from cancellation of surface charges and the latter from rapid channel block.With or without Mg2+, intracellular ADP could either stimulate or inhibit K-ATP channel activity. Low concentrations (1–100 μm) of ADP rapidly and reversibly increased average activity by a factor of 2 to 3. This activation was seen in half of the patches tested and was greater in the presence of mm Mg2+. High concentrations (>100 μm) of ADP inhibited activity with a half-block concentration of 450 μm in 0 Mg2+, i.e., more than an order of magnitude the value for ATP. ADP inhibition, like ATP inhibition, was partially relieved by mm Mg2+, suggesting that the Mg2+-bound ADP forms are less effective than free ADP forms.During exercise, free ADP levels rise and ATP declines while remaining high. Since ADP inhibition appears to compete with ATP inhibition, the two distinct processes of ADP activation and ADP inhibition could therefore promote opening of K-ATP channels during intense muscle work.

β2-Adrenergic Ion-Channel Coupled Receptors as Conformational Motion Detectors

Ion Channel-Coupled Receptors (ICCRs) are artificial proteins comprised of a G protein-coupled receptor and a fused ion channel, engineered to couple channel gating to ligand binding. These novel biological objects have potential use in drug screening and functional characterization, in addition to providing new tools in the synthetic biology repertoire as synthetic K+-selective ligand-gated channels. The ICCR concept was previously validated with fusion proteins between the K+ channel Kir6.2 and muscarinic M2 or dopaminergic D2 receptors. Here, we extend the concept to the distinct, longer β2-adrenergic receptor which, unlike M2 and D2 receptors, displayed barely detectable surface expression in our Xenopus oocyte expression system and did not couple to Kir6.2 when unmodified. Here, we show that a Kir6.2-binding protein, the N-terminal transmembrane domain of the sulfonylurea receptor, can greatly increase plasma membrane expression of β2 constructs. We then demonstrate how engineering of both receptor and channel can produce β2-Kir6.2 ICCRs. Specifically, removal of 62–72 residues from the cytoplasmic C-terminus of the receptor was required to enable coupling, suggesting that ligand-dependent conformational changes do not efficiently propagate to the distal C-terminus. Characterization of the β2 ICCRs demonstrated that full and partial agonists had the same coupling efficacy, that an inverse agonist had no effect and that the stabilizing mutation E122 W reduced agonist-induced coupling efficacy without affecting affinity. Because the ICCRs are expected to report motions of the receptor C-terminus, these results provide novel insights into the conformational dynamics of the β2 receptor.